Selectively cross-linked thermal interface materials

ABSTRACT

A process of forming a thermal interface material structure includes selectively masking a putty pad that includes ultraviolet (UV) curable cross-linkers to form a masked putty pad. The masked putty pad has a first area that is exposed and a second area that is masked. The process also includes exposing the masked putty pad to UV light to form a selectively cross-linked putty pad. The process includes disposing the selectively cross-linked putty pad between an electrical component and a heat spreader to form an assembly. The process further includes compressing the assembly to form a thermal interface material structure that includes a selectively cross-linked thermal interface material.

BACKGROUND

In an electronic device, a thermal interface material (also referred toas a “TIM”) is a material (e.g., a grease or a putty) that is disposedbetween a heat generating component of an electronic device (e.g., adie, a memory component, an inductor, etc.) and a heat spreader in orderto facilitate efficient heat transfer between the heat generatingcomponent and the heat spreader. The powering up or powering down of theelectronic device may cause a relative motion between the heatgenerating component and the heat spreader, including in-plane motionand out-of-plane motion. This relative motion may cause the thermalinterface material to squeeze out of the interface gap. This phenomenonis commonly referred to as “pump-out” of the thermal interface materialand results in increased thermal resistance due to loss of material fromthe interface. Additionally, in the context of vertical TIMapplications, another disadvantage associated with the use ofgrease/putty thermal interface materials is that such materials have apropensity to “creep” from the interface over time, which results inincreased thermal resistance due to loss of material from the interface.

SUMMARY

According to an embodiment, a process of forming a thermal interfacematerial structure is disclosed. The process includes selectivelymasking a putty pad that includes ultraviolet (UV) curable cross-linkersto form a masked putty pad. The masked putty pad has a first area thatis exposed and a second area that is masked. The process also includesexposing the masked putty pad to UV light to form a selectivelycross-linked putty pad. The process includes disposing the selectivelycross-linked putty pad between an electrical component and a heatspreader to form an assembly. The process further includes compressingthe assembly to form a thermal interface material structure thatincludes a selectively cross-linked thermal interface material.

According to another embodiment, a process of forming a thermalinterface material structure is disclosed. The process includes applyinga grease that includes UV curable cross-linkers to a surface of anelectrical component, to a surface of a heat spreader, or a combinationthereof. The process also includes forming an assembly by compressingthe grease between the electrical component and the heat spreader. Theprocess further includes exposing the assembly to UV light to form athermal interface material structure that includes a selectivelycross-linked thermal interface material.

According to another embodiment, a thermal interface material structureis disclosed that includes a selectively cross-linked thermal interfacematerial. The selectively cross-linked thermal interface materialincludes a first area having UV curable cross-linkers in a cross-linkedstate and a second area having the UV curable cross-linkers in anon-crosslinked state.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescriptions of exemplary embodiments of the invention as illustrated inthe accompanying drawings wherein like reference numbers generallyrepresent like parts of exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a first example of a process of forminga selectively cross-linked thermal interface material, according to oneembodiment.

FIG. 2 is a diagram illustrating a second example of a process offorming a selectively cross-linked thermal interface material, accordingto one embodiment.

FIG. 3 is a flow diagram depicting a particular embodiment of a processof forming a selectively cross-linked thermal interface material.

FIG. 4 is a flow diagram depicting a particular embodiment of a processof forming a selectively cross-linked thermal interface material.

DETAILED DESCRIPTION

The present disclosure describes selectively cross-linked thermalinterface materials and processes of forming the selectivelycross-linked thermal interface materials. In the present disclosure,cross-linking moieties that are triggered using ultraviolet (UV) light(also referred to herein as “UV curable cross-linkers”) are utilized toform the selectively cross-linked thermal interface materials. In orderto reduce thermal interface material migration (e.g., resulting frompump-out and/or creep of TIM material from the interface), curing of theUV curable cross-linkers may be limited to selected areas of aninterface between an electrical component that generates heat duringelectronic device operations (e.g., a die, a memory component, aninductor, etc.) and a heat spreader. For example, exposure to UV lightmay be limited to a first area of the interface (e.g., along the outeredges of the interface) in order to initiate a cross-linking reaction inthe first area but not in a second area of the interface (e.g., in thecentral region of the interface away from the edges) that represents amajority of the interface area. Such selective exposure to UV lightresults in a thermal interface material that is cross-linked in thefirst area (also referred to herein as a “cross-linked region” or a“cured region”) and that is not cross-linked in the second area (alsoreferred to herein as an “uncured region”). The cured region is stifferand less compliant than the uncured region. For applications wherehighly compliant thermal interface materials are required, the thermalinterface materials of the present disclosure may be advantageousbecause the majority of the thermal interface material remainscompliant, while the edges are stiffened to mitigate material pump-outand/or creep.

In one embodiment, a pad of putty (also referred to herein as a “puttypad”) may be used. The putty pad (e.g., a pre-cut pad or a pad formedfrom a dispensable putty) may include a mixture of materials thatincludes UV curable cross-linkers. In some cases, a photo-initiator maybe utilized to initiate or catalyze the UV reaction. An illustrative,non-limiting embodiment of a photo-initiator that may be utilized forsilicone acrylates is 2-hydroxy-2-methylpropiophenone. It will beappreciated that an alternative photo-initiator may be selected by oneof ordinary skill in the art based on the particular UV curablecross-linkers that are used. The putty pad is masked in the second area,and the first area is then exposed to UV light such that thecross-linking reaction occurs in the first area (e.g., along the edgesof the pad) but not in the second area (e.g., the central region of thepad) to form a selectively cross-linked putty pad. The selectivelycross-linked putty pad is subsequently disposed between an electricalcomponent and a heat spreader and then compressed. In this example, thecentral region of the selectively cross-linked putty pad remainscompliant to conform to the surfaces of the die and the heat spreader,while the stiffer edges mitigate material pump-out and/or creep. WhileFIGS. 1 and 2 depict an example in which the electrical component is adie, it will be appreciated that the selectively cross-linked putty padmay be disposed between another type of electrical component (e.g., amemory component, an inductor, etc.) and the heat spreader.

In another embodiment, a grease may be utilized, where the greaseincludes a mixture of materials that includes UV curable cross-linkers.In some cases, a photo-initiator may be utilized to initiate or catalyzethe UV reaction. An illustrative, non-limiting embodiment of aphoto-initiator that may be utilized for silicone acrylates is2-hydroxy-2-methylpropiophenone. It will be appreciated that analternative photo-initiator may be selected by one of ordinary skill inthe art based on the particular UV curable cross-linkers that are used.In this case, the grease may be applied between the electrical componentand the heat spreader in order to achieve a desired bond line.Subsequently, the edges of the assembly are exposed to UV light suchthat the cross-linking reaction occurs along the edges of the assembly.After the cross-linking reaction, the stiffened edges mitigate materialpump-out and/or creep.

FIG. 1 is a diagram 100 illustrating an example of a process of forminga selectively cross-linked thermal interface material 102, according toone embodiment. The top portion of FIG. 1 illustrates that a selectivelycross-linked putty pad 104 may be formed via selective exposure of aputty pad 106 (that includes UV curable cross-linkers 108) to UV light110. The bottom portion of FIG. 1 illustrates that the selectivelycross-linked putty pad 104 may subsequently be disposed between anelectrical component, such as a die 120 or another type of electricalcomponent that generates heat during electronic device operations (e.g.,a memory component, an inductor, etc.) and a heat spreader 122 to forman assembly 124. The assembly 124 is then compressed to form a thermalinterface material structure 126 that includes the selectivelycross-linked thermal interface material 102. In this example, thecentral region of the selectively cross-linked putty pad 104 remainscompliant to conform to the surfaces of the die 120 and the heatspreader 122, while the stiffer edges mitigate material pump-out and/orcreep. For applications where highly compliant thermal interfacematerials are required, the selectively cross-linked thermal interfacematerial 102 of FIG. 1 may be advantageous because the majority of thethermal interface material remains compliant, while the edges arestiffened to mitigate material pump-out and/or creep.

Referring to the top portion of FIG. 1, the putty pad 106 that includesthe UV curable cross-linkers 108 is selectively masked to form a maskedputty pad 112, and the masked putty pad 112 is subsequently exposed to aparticular wavelength of UV light to form the selectively cross-linkedputty pad 104. FIG. 1 illustrates that the putty pad 106 has a shapethat is appropriate for the particular features of the die 120 and theheat spreader 122 depicted at the bottom of FIG. 1. The putty pad 106may include a pre-cut pad having the appropriate shape or may be formedfrom dispensable putty into the appropriate shape.

In some embodiments, the UV curable cross-linkers 108 of the putty pad106 may include epoxy, acrylate, vinyl, dimers, or other UV responsivemoieties. For illustrative purposes only, FIG. 1 depicts an embodimentin which the UV curable cross-linkers 108 include molecules that mayundergo a photodimerization reaction, with the resulting dimerrepresenting an example of a cross-linked material. In a particularembodiment, the putty pad 106 may include a silicone material, and theUV curable cross-linkers 108 may be covalently bonded to the siliconematerial. In some embodiments, the UV curable cross-linkers 108 mayinclude a compound (e.g., coumarin, in the illustrative example ofFIG. 1) that may reversibly cross-link via a reversiblephotodimerization reaction. In such cases, when the compound is exposedto UV light of a particular wavelength (e.g., above 350 nm in the caseof coumarin), the compound undergoes a photodimerization reaction (forselective cross-linking). While not shown in the example of FIG. 1,subsequent exposure to UV light of a different wavelength (e.g., lessthan 260 nm, in the case of coumarin) may reverse the cross-linking anddimerization, returning to the original compound (a non-crosslinkedstate).

Examples of such compounds include coumarin, anthracene, cinnamic acid,thymine, and stilbene (among numerous other alternatives). Toillustrate, a coumarin compound may undergo a dimerization reaction whenexposed to UV light above 350 nm, and the dimerization reaction may bereversed via exposure to UV light below 260 nm. An anthracene compoundmay undergo a dimerization reaction when exposed to UV light above 350nm, and the dimerization reaction may be reversed via exposure to UVlight below 300 nm. A cinnamic acid compound may undergo a dimerizationreaction when exposed to UV light above 300 nm, and the dimerizationreaction may be reversed via exposure to UV light below 260 nm. Athymine compound may undergo a dimerization reaction when exposed to UVlight above 300 nm, and the dimerization reaction may be reversed viaexposure to UV light below 260 nm. A stilbene compound may undergo adimerization reaction when exposed to UV light above 300 nm, and thedimerization reaction may be reversed via exposure to UV light below 260nm.

A typical thermal interface material “rework” process entails thereplacement of pads or removal of material. Selection of the UV curablecross-linkers 108 that have the ability to undergo reversiblephotodimerization may enable the selectively cross-linked thermalinterface material 102 to be reversed back to its original state forreworking operations and selectively cured again to retain its thermalintegrity resistance. In contrast to typical thermal interface reworkprocedures, utilizing the selectively cross-linked thermal interfacematerial 102 of the present disclosure may enable rework operations tobe performed without potential device damage associated with materialremoval while also preventing material waste. Further, the reversiblenature of the photodimerization reaction may enable tuning of thethermal interface material properties. The ability to tune the thermalinterface material properties may be beneficial for rework, changingoperating environments, or operating in harsh environments, among otherbenefits.

Referring to the middle of the top portion of FIG. 1, a mask 114 isapplied to the putty pad 106 to form the masked putty pad 112. Afterapplication of the mask 114 to the putty pad 106, FIG. 1 illustratesthat a first area 116 (identified as “Area(1)” in FIG. 1) of the maskedputty pad 112 remains unmasked for subsequent exposure to the UV light110, and the mask 114 prevents a second area beneath the mask 114 frombeing exposed to the UV light 110. In the example depicted in FIG. 1,the first area 116 corresponds to the edges of the putty pad 106, whilethe second area corresponds to the central region and represents amajority of the surface area of the putty pad 106. Referring to theright side of the top portion of FIG. 1, the masked putty pad 112 isthen exposed to the UV light 110 such that the cross-linking reactionoccurs in the first area 116 (e.g., along the pad edges). The mask 114may then be removed to form the selectively cross-linked putty pad 104.

The bottom portion of FIG. 1 shows that the selectively cross-linkedputty pad 104 may be used to form the selectively cross-linked thermalinterface material 102. Referring to the left side of the bottom portionof FIG. 1, the selectively cross-linked putty pad 104 may be disposedbetween the die 120 and the heat spreader 122 to form the assembly 124.Referring to the right side of the bottom portion of FIG. 1, theassembly 124 may be compressed to form the thermal interface materialstructure 126 that includes the selectively cross-linked thermalinterface material 102.

Thus, FIG. 1 illustrates an example of a process of forming aselectively cross-linked thermal interface material, according to oneembodiment. In the example of FIG. 1, the selectively cross-linkedthermal interface material is formed from a selectively cross-linkedputty pad that is disposed between an electrical component thatgenerates heat during electronic device operations (e.g., a die) and aheat spreader and then compressed. In this example, the central regionof the selectively cross-linked putty pad remains compliant to conformto the surfaces of the electrical component and the heat spreader, whilethe stiffer edges mitigate material pump-out and/or creep. Forapplications where highly compliant thermal interface materials arerequired, the selectively cross-linked thermal interface material ofFIG. 1 may be advantageous because the majority of the thermal interfacematerial remains compliant, while the edges are stiffened to mitigatematerial pump-out and/or creep. Further, in some cases, the selectivelycross-linked thermal interface material of FIG. 1 may include compoundsthat undergo reversible photodimerization when exposed to UV light. Suchcompounds may enable TIM rework operations to be performed without thepotential device damage and material waste associated with a typical TIMrework process that entails the replacement of pads or removal ofmaterial, among other benefits.

Referring to FIG. 2, a diagram 200 illustrates an example of a processof forming a selectively cross-linked thermal interface material 202,according to one embodiment. In contrast to the putty pad 106 of FIG. 1,the top portion of FIG. 2 illustrates that a grease 206 (that includesUV curable cross-linkers 208) may be applied to a surface of anelectrical component, such as a die 220, (and/or a surface of a heatspreader 222) in order to achieve a desired bond line. The bottomportion of FIG. 2 illustrates that the grease 206 may be compressedbetween the die 220 and the heat spreader 222 to form an assembly 224.Subsequently, the edges of the assembly 224 are exposed to UV light 210such that the cross-linking reaction occurs along the edges of theassembly 224. After the cross-linking reaction, the stiffened edgesmitigate material pump-out and/or creep. For applications where highlycompliant thermal interface materials are required, the selectivelycross-linked thermal interface material 202 of FIG. 2 may beadvantageous because the majority of the thermal interface materialremains compliant, while the edges are stiffened to mitigate materialpump-out and/or creep.

Referring to the left side of the top portion of FIG. 2, the grease 206may include a mixture of materials that includes the UV curablecross-linkers 208. In some embodiments, the UV curable cross-linkers 208may include epoxy, acrylate, vinyl, dimers, or other UV responsivemoieties. For illustrative purposes only, FIG. 2 depicts an embodimentin which the UV curable cross-linkers 208 include molecules that mayundergo a photodimerization reaction, with the resulting dimerrepresenting an example of a cross-linked material. In a particularembodiment, the grease 206 may include a silicone material, and the UVcurable cross-linkers 208 may be covalently bonded to the siliconematerial. In some embodiments, the UV curable cross-linkers 208 mayinclude a compound (e.g., coumarin, in the illustrative example of FIG.2) that may reversibly cross-link via a reversible photodimerizationreaction. As previously described herein with respect to FIG. 1,examples of such compounds include coumarin, anthracene, cinnamic acid,thymine, and stilbene (among numerous other alternatives).

Selection of the UV curable cross-linkers 208 that have the ability toundergo reversible photodimerization may enable the selectivelycross-linked thermal interface material 202 to be reversed back to itsoriginal state for reworking operations and selectively cured again toretain its thermal resistance integrity. In contrast to typical thermalinterface rework procedures, utilizing the selectively cross-linkedthermal interface material 202 of FIG. 2 may enable rework operations tobe performed without potential device damage associated with materialremoval while also preventing material waste. Further, the reversiblenature of the photodimerization reaction may enable tuning of thethermal interface material properties. The ability to tune the thermalinterface material properties may be beneficial for rework, changingoperating environments, or operating in harsh environments, among otherbenefits.

The bottom portion of FIG. 2 shows the formation of the selectivelycross-linked thermal interface material 202 after application of thegrease 206. Referring to the left side of the bottom portion of FIG. 2,the grease 206 is disposed between the die 220 and the heat spreader222, followed by compression to form the assembly 224. Referring to theright side of the bottom portion of FIG. 2, the assembly 224 may beexposed to UV light 210 to form a thermal interface material structure226 that includes the selectively cross-linked thermal interfacematerial 202.

Thus, FIG. 2 illustrates an example of a process of forming aselectively cross-linked thermal interface material, according to oneembodiment. In the example of FIG. 2, the selectively cross-linkedthermal interface material is formed from a grease that includes amixture of materials that includes UV curable cross-linkers. FIG. 2illustrates that the grease may be applied between an electricalcomponent that generates heat during electronic device operations (e.g.,a die) and the heat spreader in order to achieve the desired bond line.Subsequently, the edges of the assembly are exposed to UV light suchthat the cross-linking reaction occurs along the edges of the assemblyto form the selectively cross-linked thermal interface material. Forapplications where highly compliant thermal interface materials arerequired, the selectively cross-linked thermal interface material ofFIG. 2 may be advantageous because the majority of the thermal interfacematerial remains compliant, while the edges are stiffened to mitigatematerial pump-out and/or creep. Further, in some cases, the selectivelycross-linked thermal interface material of FIG. 2 may include compoundsthat undergo reversible photodimerization when exposed to UV light. Suchcompounds may enable TIM rework operations to be performed without thepotential device damage and material waste associated with a typical TIMrework process that entails the replacement of pads or removal ofmaterial, among other benefits.

Referring to FIG. 3, a flow diagram illustrates an example of a process300 of forming a selectively cross-linked thermal interface material,according to one embodiment. In the example of FIG. 3, the selectivelycross-linked thermal interface material is formed from a selectivelycross-linked putty pad that includes UV curable cross-linkers. A centralregion of the selectively cross-linked putty pad remains compliant toconform to surfaces of an electrical component that generates heatduring electronic device operations (e.g., a die, a memory component, aninductor, etc.) and a heat spreader, while the stiffer edges mitigatematerial pump-out and/or creep. Such properties may be particularlyadvantageous in applications where highly compliant thermal interfacematerials are required. It will be appreciated that the operations shownin FIG. 3 are for illustrative purposes only and that the operations maybe performed in alternative orders, at alternative times, by a singleentity or by multiple entities, or a combination thereof. For example,one entity may form a putty pad (e.g., a pre-cut pad or a pad that isformed from dispensable putty), while the same entity or a differententity may form the masked putty pad (illustrated as operation 302 inFIG. 3). In some cases, the same entity or a different entity may exposethe masked putty pad to UV light to form a selectively cross-linkedputty pad (illustrated as operation 304 in FIG. 3). Additionally, thesame entity or a different entity may form a thermal interface materialstructure that includes a selectively cross-linked thermal interfacematerial by compressing the selectively cross-linked putty pad betweenan electrical component and a heat spreader (illustrated as operations306 and 308 in FIG. 3).

The process 300 includes selectively masking a putty pad that includesUV curable cross-linkers to form a masked putty pad, at 302. The maskedputty pad has a first area that is exposed and a second area that isdisposed beneath the mask. For example, referring to FIG. 1, the puttypad 106 that includes the UV curable cross-linkers 108 may be used toform the masked putty pad 112. The mask 114 that is applied to the puttypad 106 has a shape that is selected such that a majority of the surfacearea of the putty pad 106 is not exposed to the UV light 110. In theexample of FIG. 1, the putty pad 106 has a square/rectangular shape, andthe first area 116 that is not covered by the mask 114 corresponds tothe horizontal and vertical edges of the putty pad 106. It will beappreciated that alternative putty pad shapes may be utilized, and theexposed areas may have alternative shapes and/or sizes that areappropriate for a particular combination of a die and a heat spreader.

The process 300 includes exposing the masked putty pad to UV light toform a selectively cross-linked putty pad, at 304. For example,referring to FIG. 1, the masked putty pad 112 is exposed to the UV light110 to form the selectively cross-linked putty pad 104. As describedpreviously herein, the UV curable cross-linkers 108 may include avariety of materials, and a particular wavelength of the UV light 110 isappropriate to initiate a cross-linking reaction for the particularmaterial(s). In the illustrative, non-limiting example depicted in FIG.1, the UV curable cross-linkers 108 include a coumarin material thatundergoes reversible photodimerization when exposed to UV light having awavelength that is greater than 350 nm.

The process 300 includes disposing the selectively cross-linked puttypad between an electrical component (e.g., a die, a memory component, aninductor, etc.) and a heat spreader to form an assembly, at 306. Forexample, referring to the bottom portion of FIG. 1, the selectivelycross-linked putty pad 104 may be disposed between the die 120 and theheat spreader 122 to form the assembly 124.

The process 300 includes compressing the assembly to form a thermalinterface material structure that includes a selectively cross-linkedthermal interface material, at 308. For example, referring to FIG. 1,the assembly 124 may be compressed to form the thermal interfacematerial structure 126 that includes the selectively cross-linkedthermal interface material 102.

While not shown in the example of FIG. 3, in cases where the UV curablecross-linkers 108 have the ability to undergo reversiblephotodimerization, the process 300 may further include exposing theselectively cross-linked thermal interface material 102 to UV light ofan appropriate wavelength to reverse the dimerization reaction. Forexample, in the illustrative, non-limiting example depicted in FIG. 1,the photodimerization reaction may be reversed when the selectivelycross-linked thermal interface material 102 is exposed to UV lighthaving a wavelength that is less than 260 nm. As previously describedherein, the ability to reverse the cross-linking reaction may enablerework operations to be performed without the potential device damageand material waste associated with typical thermal interface reworkprocedures, among other benefits.

Thus, FIG. 3 illustrates an example of a process of forming aselectively cross-linked thermal interface material. For applicationswhere highly compliant thermal interface materials are required, theselectively cross-linked thermal interface material formed according tothe process depicted in FIG. 3 may be advantageous because the majorityof the thermal interface material remains compliant, while the edges arestiffened to mitigate material pump-out and/or creep.

Referring to FIG. 4, a flow diagram illustrates an example of a process400 of forming a selectively cross-linked thermal interface material,according to one embodiment. In the example of FIG. 4, the selectivelycross-linked thermal interface material is formed from a grease thatincludes UV curable cross-linkers. After application of the grease to asurface of an electrical component, such as a die, and/or a surface of aheat spreader in order to achieve a desired bond line, the grease may becompressed between the die and the heat spreader to form an assembly.Subsequently, the assembly is exposed to UV light in order to initiate across-linking reaction at the exposed edges of the assembly. Thestiffened cross-linked material at the edges of the assembly maymitigate material pump-out and/or creep, while the majority of thethermal interface material remains compliant. Such properties may beparticularly advantageous in applications where highly compliant thermalinterface materials are required. It will be appreciated that theoperations shown in FIG. 4 are for illustrative purposes only and thatthe operations may be performed in alternative orders, at alternativetimes, by a single entity or by multiple entities, or a combinationthereof. For example, one entity may form a grease that includes UVcurable cross-linkers, while another entity may apply the grease to asurface of the electrical component and/or a surface of a heat spreaderprior to forming an assembly by compressing the grease between theelectrical component and the heat spreader (illustrated as operations402 and 404 in FIG. 4). In some cases, the same entity or a differententity may expose the assembly to UV light to form a thermal interfacematerial structure that includes a selectively cross-linked thermalinterface material (illustrated as operation 406 in FIG. 4).

The process 400 includes applying a grease that includes UV curablecross-linkers to a surface of an electrical component (e.g., a die)and/or a surface of a heat spreader, at 402. For example, referring tothe top portion of FIG. 2, the grease 206 (that includes the UV curablecross-linkers 208) may be applied to the surface of the die 220. Whilenot shown in the example illustrated in FIG. 2, the grease 206 mayalternatively and/or additionally be applied to a surface of the heatspreader 222.

The process 400 includes forming an assembly by compressing the greasebetween the electrical component and the heat spreader, at 404. Forexample, referring to the bottom portion of FIG. 2, after application ofthe grease 206 to the surface of the die 220, the grease 206 may becompressed between the die 220 and the heat spreader 222 to form theassembly 224.

The process 400 includes exposing the assembly to UV light to form athermal interface material structure that includes a selectivelycross-linked thermal interface material, at 406. For example, referringto FIG. 2, exposure of the assembly 224 to the UV light 210 may initiatea cross-linking reaction along the exposed edges of the assembly 224. Asdescribed previously herein, the UV curable cross-linkers 208 mayinclude a variety of materials, and a particular wavelength of the UVlight 210 is appropriate to initiate a cross-linking reaction for theparticular material(s). In the illustrative, non-limiting exampledepicted in FIG. 2, the UV curable cross-linkers 208 include a coumarinmaterial that undergoes reversible photodimerization when exposed to UVlight having a wavelength that is greater than 350 nm.

While not shown in the example of FIG. 4, in cases where the UV curablecross-linkers 208 have the ability to undergo reversiblephotodimerization, the process 400 may further include exposing theselectively cross-linked thermal interface material 202 to UV light ofan appropriate wavelength to reverse the dimerization reaction. Forexample, in the illustrative, non-limiting example depicted in FIG. 2,the photodimerization reaction may be reversed when the selectivelycross-linked thermal interface material 202 is exposed to UV lighthaving a wavelength that is less than 260 nm. As previously describedherein, the ability to reverse the cross-linking reaction may enablerework operations to be performed without the potential device damageand material waste associated with typical thermal interface reworkprocedures, among other benefits.

Thus, FIG. 4 illustrates an example of a process of forming aselectively cross-linked thermal interface material. For applicationswhere highly compliant thermal interface materials are required, theselectively cross-linked thermal interface material formed according tothe process depicted in FIG. 4 may be advantageous because the majorityof the thermal interface material remains compliant, while the edges arestiffened to mitigate material pump-out and/or creep.

It will be understood from the foregoing description that modificationsand changes may be made in various embodiments of the present inventionwithout departing from its true spirit. The descriptions in thisspecification are for purposes of illustration only and are not to beconstrued in a limiting sense. The scope of the present invention islimited only by the language of the following claims.

What is claimed is:
 1. A process of forming a selectively cross-linkedputty pad, the process comprising: selectively masking a putty pad thatincludes ultraviolet (UV) curable cross-linkers to form a masked puttypad, the masked putty pad having a first area that is exposed and asecond area that is masked; and forming a selectively cross-linked puttypad by exposing the masked putty pad to UV light, the selectivelycross-linked putty pad having the UV curable cross-linkers in across-linked state in the first area and in a non-crosslinked state inthe second area.
 2. The process of claim 1, wherein the UV curablecross-linkers include compounds that reversibly cross-link via areversible photodimerization reaction.
 3. The process of claim 2,wherein the UV curable cross-linkers include a coumarin compound, ananthracene compound, a cinnamic acid compound, a thymine compound, or astilbene compound.
 4. The process of claim 2, wherein the putty padincludes a silicone material, and wherein the UV curable cross-linkersinclude a coumarin compound that is covalently bonded to the siliconematerial.
 5. The process of claim 1, further comprising: disposing theselectively cross-linked putty pad between an electrical component and aheat spreader; and compressing the electrical component and the heatspreader.
 6. The process of claim 5, wherein the putty pad includes apre-cut pad having an appropriate shape for the electrical component. 7.The process of claim 5, wherein the putty pad is formed from dispensableputty into an appropriate shape for the electrical component.
 8. Theprocess of claim 1, wherein the first area is an area along edges of theputty pad.
 9. The process of claim 1, wherein the second area is an areain a central region of the putty pad.
 10. The process of claim 1,wherein the UV curable cross-linkers include epoxy, acrylate, vinyl,dimers, or other UV responsive moieties.
 11. The process of claim 1,wherein the UV curable cross-linkers include compounds that reversiblycross-link via a reversible photodimerization reaction.
 12. The processof claim 11, wherein the UV curable cross-linkers include a coumarincompound, an anthracene compound, a cinnamic acid compound, a thyminecompound, or a stilbene compound.
 13. The process of claim 11, whereinthe putty pad includes a silicone material, and wherein the UV curablecross-linkers include a coumarin compound that is covalently bonded tothe silicone material.